In contemporary radio based communications systems, frequency modulation (FM) is often used to communicate information including voice and data wirelessly. Since the communicated information is represented in the form of absolute frequency, it is paramount that the frequency stability of the oscillator is well controlled. Many communications system operate at several hundred MHz. Frequency stability is often difficult to achieve economically at these relatively high operating frequencies. Furthermore, for efficiency reasons, it is also desirable to have a wide span of FM control of the oscillator. A common measure of the span of control is often called "pullability" of the oscillator, and refers to an extent that an oscillator can be modulated. The requirements for frequency stability, high operating frequency, and pullability are often competing. As a result using an oscillator architecture that has good frequency stability often means that it needs to be operated at fundamental mode. Conversely, an oscillator with good pullability generally has relatively poor frequency stability, but may be operable at a high operating frequency.
Often, to achieve acceptable frequency stability, crystal based oscillators are used. One difficulty with a crystal oscillator is that it is difficult at best to construct a frequency stable and pullable oscillator operating at frequencies above about 30 MHz using an A-T cut crystal. To achieve higher operating frequencies, frequency multipliers are often employed to scale the oscillator's frequency upwards. It is not uncommon to stage several frequency multipliers in series to further boost the oscillator's frequency. A problem with this approach is additional complexity because of the additional frequency multiplier stages which leads to higher complexity, difficulty of manufacturing, and lower field reliability, not to mention higher cost.
Another approach is to use an overtone oscillator that oscillates at overtone frequencies rather than at a crystal's fundamental frequency. So, a third overtone 25 MHz fundamental oscillator will operate at 75 MHz. This approach requires less frequency scaling which reduces complexity. However the pullability suffers dramatically as the overtone order increases. As a practical matter the pullability decreases because the crystal's motional capacitance decreases inversely as square of the overtone order. So, for a 3rd order overtone oscillator the motional capacitance decreases by a factor of 9, and for a 5th order overtone the motional capacitance decreases by a factor of 25.
FIG. 1 illustrates a typical prior art pullable crystal oscillator in a Colpitts arrangement 101 operating at 25 MHz with two post frequency multiplier/tripler stages 103, 105 that raise the oscillator's frequency from 25 MHz to 75 MHz, and then to 225 MHz. Pullability is achieved by forcing a modulation signal into the circuit at reference number 107.
This scheme performs acceptably for many applications but suffers from complexity, difficulty of manufacturing, and other factors introduced earlier. As mentioned earlier a 3rd overtone oscillator could be substituted for the oscillator 101 and the first frequency tripler 103 but the system's pullability would suffer significantly.
What is needed is an improved frequency modulable oscillator that is stable, operates at a relatively high frequency and has good pullability using a simpler structure that is more reliable, easier to manufacture, and system implementation that is less costly.